1. The Field of the Invention
The present invention relates generally to paving binder compositions and methods for manufacturing such compositions. More specifically, the present invention relates to high sulfur paving binder compositions that contain a sulfur plasticizer such as asphalt and which retain non-flow properties within a broad range of ambient temperatures, and to methods for producing binders with these compositions.
2. Present State of the Art
Paving material typically includes a binder and an aggregate. Although the binder is typically the minority component in paving materials, most of the pavement properties that relate to its longevity depend on the properties of the binder.
The binder component is generally an asphalt-based composition that may include some additives. Asphalt is described as a dark brown to black cementitious material, which has a solid, semisolid or liquid consistency, in which the predominant constituents are bitumens that occur in nature as such or which are obtained as residue in refining petroleum. Natural deposits in which asphalt occurs within porous rocks are known as rock asphalts. Petroleum asphalt is part of the residue that is obtained in the distillation of petroleum. In particular, asphalt cement is petroleum asphalt that is refined to meet specifications for paving, industrial, and special purposes.
The aggregate component of paving material is typically any hard, inert, mineral material that is used for mixing in graduated fragments. The aggregate component may include sand, gravel, crushed stone, coral, and slag.
One of the limitations to the use of asphalt as a binder for paving materials is that it softens and flows within a wide range of ambient temperatures. This limitation makes the transport of this type of conventional asphalt-based materials difficult or even impossible, and it can also give rise to serious environmental problems. Nevertheless, convenient transport of binder materials is desirable because paving takes place at sites that are generally far away from the locations where the asphalt is available. Transporting binder materials in the form of smaller-sized, solid, non-sticky and non-flowing materials facilitates the delivery of paving binders even when they are transported to sites that are far away from the asphalt sources.
Asphalt and sulfur are used in the manufacture of binders and they are typically transported in liquid form. This type of transport requires specialized containers and conditions. It is desirable to manufacture binders that incorporate the required amounts of asphalt and sulfur and they remain in solid, non-sticky and non-flowing form over a wide range of ambient temperatures, so that such binders can be conveniently transported over long distances by conventional means of transportation for common solids.
Asphalt additives are used to render the binder material less flowable at ambient temperatures. In particular, sulfur is one of such additives that has been incorporated into the binder as a minority binder constituent. Mixing asphalt with sulfur, however, presents a number of problems. Some of these problems derive from the different densities of asphalt and sulfur. In certain mixtures, sulfur separates due to its greater density from the asphalt. As a result, the sulfur depleted binder then retains the softening and flowing properties of asphalt, which make the handling and transportation of the binder difficult or impossible. Finely divided calcium-based materials have also been used in an attempt to keep the sulfur additive homogeneously dispersed in asphalt. For example, crushed limestone has been used for this purpose. However, the use of calcium-based materials is believed to lead to the formation of calcium sulfides and polysulfides that are detrimental to the pavement longevity.
Furthermore, it has been acknowledged that sulfur-rich binders may detrimentally affect the quality and longevity of the paving material. In addition, sulfur has been viewed as a constituent that would unacceptably increase the cost of binder materials to the point of rendering them prohibitively expensive if the amount of sulfur in the binder exceeded a certain limit. For example, the sulfur concentration by weight in binders is not significantly above 50%, and the art recognizes that a sulfur-to-asphalt weight ratio greater than 1.5:1 increases the cost and may make the binder so sulfur-rich as to detrimentally affect the asphalt. Unless indicated otherwise, concentrations given as percentages are hereinafter understood as being weight percentages. Furthermore, sulfur-asphalt mixtures that contain more than 52% of sulfur are conventionally described as being too sensitive to compaction temperatures below 115.5xc2x0 C. (240xc2x0 F.).
In addition to economic considerations regarding the use of sulfur as an additive in paving binders, the use of asphalt is also related to economic factors. For example, the use of asphalt as the major constituent in paving binders is negatively affected by the often fluctuating petroleum production patterns. Further, limited petroleum supplies may threaten, in the long term, the viability of paving binders in which asphalt is a major constituent. Profitable utilization of petroleum products is another factor that detrimentally affects the use of asphalt as a majority constituent in paving binders. For example, maintaining, renovating and protecting the surfaced highways and streets in the United States requires approximately thirty (30) million tons of asphalt cement annually. Asphalt cement was available in the past at a reasonable cost because asphalt cement is a residue in petroleum refining and certain petroleum refining residues could only be economically utilized for the production of asphalt cement. However, higher percentages of petroleum are utilized nowadays for the production of other more profitable forms of petroleum products, such as petroleum coke. As this trend continues, the price of asphalt cement is expected to increase even under constant demand. This expectation is supported by the evolution of the average price of asphalt cement over the past thirty-two (32) years, a period during which the price has risen from approximately $23/ton in 1968 to approximately $152/ton in 2000 (through February), an increase of about 561%. It is generally recognized, however, that there is currently no economical paving binder that can be substituted for asphalt cement, and that there is no low asphalt paving binder that can effectively replace high asphalt paving binders.
The use of sulfur often leads to acridity problems because the smell of sulfur is generally considered unpleasant. This is so even when sulfur applications do not lead to the formation of products such as sulfides and sulfur oxides.
Because of its color, the addition of sulfur to mixtures such as binders causes the manufactured binder to be paler than it would otherwise be without sulfur. It is, however, desirable for some applications to control the color of the binder. The ability to control the binder""s color permits the production of a binder that can satisfy a wider range of customer expectations. For example, some customers expect paved surfaces to be of a certain color for aesthetic purposes. Other customers expect paved surfaces to offer a certain appearance by displaying a color that is viewed as harmonious with respect to other environmental factors.
Paved surfaces are typically exposed during long time periods to solar radiation. Certain types of such radiation cause the chemical transformation of binder components, and thus the degradation of the pavement in which such binder is incorporated. An example of such radiation is ultraviolet radiation, the exposure to which causes asphalt embrittlement.
Accordingly, there is a need for paving binders which include the following characteristics. First, these binders can be manufactured in forms that are non-sticky and non-flowing within a wide range of ambient temperatures at which storage and transport is effectuated. Binders with these non-sticky and non-flowing properties can be conveniently transported over long distances while avoiding pollution problems that would derive from the emissions and spills of other forms of binders that soften and flow at ambient temperatures. Second, asphalt in these paving binders should be incorporated at most as a minority component to reduce petroleum dependency and cost. Third, the additives used in the paving binders should not substantially incorporate constituents that, whether directly or when combined with other binder constituents, are known to detrimentally affect the quality and longevity of the pavement. Fourth, effects derived from the use of sulfur, such as acridity and discoloration, should be controlled by the use of appropriate binder components. Fifth, harmful radiation protection should be provided to prevent or reduce asphalt embrittlement. Finally, paving binders should include additives which are not obtained at the cost of depleting resources that can be used for other purposes, but which absorb waste substances that would otherwise present disposal problems. The foregoing characteristics should be achieved by the use of constituents that are not incompatible amongst themselves and such that the constituents"" combined properties preferably enhance the binder properties.
The present invention has been developed in response to the present state of the art and, in particular, in response to problems and needs that have not been solved heretofore.
In accordance with the invention as embodied and broadly described herein, paving binder compositions according to the present invention comprise sulfur at a concentration of not less than 60%, a carbon-based plasticizer such as asphalt, and a fine mineral constituent such as fine silica material, fly ash and mixtures thereof. Some embodiments of the paving binder according to the present invention comprise a hydrocarbon-based plasticizer that includes a mixture of asphalt and at least one organic additive. Furthermore, some embodiments of the paving binder according to the present invention comprise a fine mineral constituent that is essentially free of low density calcium-based filler, such as calcium oxide or calcium hydroxide. According to the present invention paving binders are manufactured by mixing at least sulfur, a hydrocarbon-based plasticizer and a fine mineral constituent and solidifying the fluid mixture to form paving binders in any one of a plurality of forms such as pastilles and slates.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description, drawings, and appended claims, or may be learned by the practice of the invention as set forth hereinafter.